Synthesis of neodymium ferrite incorporated graphitic carbonitride (NdFe2O4@g-C3N4) and its application in the photodegradation of ciprofloxacin and ampicillin in a water system

Purification of antibiotic-contaminated drinking water sources is faced with limitations. Therefore, this study incorporated neodymium ferrite (NdFe2O4) in graphitic carbonitride (g-C3N4) to form NdFe2O4@g-C3N4 as a photocatalyst for removing ciprofloxacin (CIP) and ampicillin (AMP) from aqueous systems. X-ray diffraction (XRD) revealed a crystallite size of 25.15 nm for NdFe2O4 and 28.49 nm for NdFe2O4@g-C3N4. The bandgap is 2.10 and 1.98 eV for NdFe2O4 and NdFe2O4@g-C3N4, respectively. The transmission electron micrograph (TEM) images of NdFe2O4 and NdFe2O4@g-C3N4 gave an average particle size of 14.10 nm and 18.23 nm, respectively. Scanning electron micrograph (SEM) images showed heterogeneous surfaces with irregular-sized particles suggesting agglomeration at the surfaces. NdFe2O4@g-C3N4 (100.00 ± 0.00% for CIP and 96.80 ± 0.80% for AMP) exhibited better photodegradation efficiency towards CIP and AMP than NdFe2O4 (78.45 ± 0.80% for CIP and 68.25 ± 0.60% for AMP) in a process described by pseudo-first-order kinetics. NdFe2O4@g-C3N4 showed a stable regeneration capacity towards degradation of CIP and AMP with a capacity that is above 95% even at the 15th cycle of treatment. The use of NdFe2O4@g-C3N4 in this study revealed its potential as a promising photocatalyst for removing CIP and AMP in water systems.


Introduction
The provision of clean drinking water is challenged with contamination from emerging and unexpected pollutants. 1,2 The unexpected contaminants in water are of different forms, most of which are classied as unregulated pollutants in water systems. Among the unregulated contaminants in water, antibiotics are of serious concern. They have been transferred by leaching into environmental water systems such as surface and underground water systems, which can serve as drinking water sources. 3 Authors have detected antibiotics in different drinking water sources, [4][5][6] and due to the current challenge of incomplete removal of antibiotics in treated drinking water sources, antibiotic contaminated water sources have become worrisome. A recent study has reported the presence of antibiotics in bottled water and other drinking water sold on the market. 7,8 Unfortunately, when antibiotics are exposed to environmental water systems, they can become metamorphosed into new forms that are more dangerous than the primary antibiotics. The harmful metabolites or metamorphoses of the antibiotics may threaten human health as they can cause diseases. 9,10 Survival of antibiotics in environmental water systems has been attributed to the emergence of antibiotic-resistance strains of microorganisms, 10,11 which compromises the efficacy of the antibiotics. It is essential to develop means of completely removing antibiotics from drinking water sources, which is the focus of this study.
Ciprooxacin (CIP) and ampicillin (AMP) are common antibiotics found in drinking water sources (surface and underground water systems). CIP was detected in Baghdad City, Iraq's water treatment plant. 7 Moreover, a high level of CIP has been reported from groundwater in South India, 12 while AMP was reported in water samples collected from the treatment plant's secondary effluent drinking water, river, groundwater, and lagoon water. 13 Recently, a study revealed high frequency (64.5% strains) of resistance to AMP in the Białka River discharge treatment resource. 14 Such contamination with AMP has a signicant environmental impact that cannot be overlooked. Moreover, discharge from such treatment plants may enter surface and underground water systems if not properly managed. Another study has shown that veterinary use is one of the signicant contributors of antibiotics to environmental water systems resulting in the contamination of drinking water sources. 15 Even though they exist in trace amounts, they negatively impact the environment and human beings. Antibiotics can adsorb solid substances in the environment and become transported over an extended range of distances. They have varying photostability and water solubility and are susceptible to biodegradation which may transform them into ecounfriendly products. 16,17 This study focuses on CIP and AMP as antibiotics of interest because of their prevalence and their frequent occurrence in drinking water sources globally. They are commonly found in drinking water sources because they are affordable and can be purchased from local pharmacy stores without prescription from physicians, especially in developing countries. They are among the most frequently used antibiotics for diseases treatment with inevitable presence in environmental water system used as drinking water sources. They have been reported across the globe in environmental drinking water sources. Therefore, it is important that an efficient method is developed to remove CIP and AMP completely from water system before they cause any havoc.
Most antibiotics used in animal and human treatment may become metabolized to different degrees, and when excreted, they remain unchanged in their active forms. 14 Although the amount is too low to impose potency against pathogenic organisms, they affect the pathogens' genetic identity, creating identity resistance against known antibiotics which is a serious public health problem. 18,19 The continuous introduction of antibiotics to the environment has led to a severe threat to public health, which has been of global concern. It has necessitated the need to develop an efficient technique for removing antibiotics such as CIP and AMP from the water system before they get into drinking water sources or during water treatment. Since CIP and AMP are common in drinking water sources, this study shall focus on developing an efficient technique for their removal in a water system. This is essential to avoid the danger that the presence of CIP and AMP may cause in water.
Several approaches have been used to remove CIP and AMP in water systems, but most have limitations. Among these limitations are high process costs and incomplete removal of CIP and AMP during treatment. Therefore, this study proposes using a nanomaterial composite as a photocatalyst for removing CIP and AMP in water. Nanocomposites have played an essential role as photocatalysts for degrading organic molecules. 20 The multi-component nature of nanocomposite gives it an advantage for functionality enhancement over singlecomponent material for photocatalysis. Ferrites are an example of multi-component materials with the possibility of being photocatalytic. The unique properties of ferrites, such as small size, thermal stability, simple synthesis route, magnetism, photosensitivity, and conductivity, give it application in water treatment. Although many photocatalysts have been reported but most of them are only active in the ultraviolet (UV) region of the light spectrum. Therefore, it becomes necessary to provide a UV light source for them to be active since UV light is not abundantly available in nature. Unfortunately, the provision of UV light source is an additional cost which increases the cost of photocatalysis process. Fortunately, ferrite such as NdFe 2 O 4 is expected to be active in the visible light region, which is advantageous for photocatalytic process. A previous study has reported the use of 3% CdS QDs/CaFe 2 O 4 @ZnFe 2 O 4 as visible light active photocatalyst for the degradation of noroxacin. 21 Tetracycline and CIP have been reportedly degraded with the use of p-CaFe 2 O 4 @n-ZnFe 2 O 4 which is a visible light active photocatalyst, however, the degradation is less than 100%. 22 Since visible light is abundant in nature and freely available at no cost, using NdFe 2 O 4 as a visible light active photocatalyst for the degradation of antibiotics (CIP and AMP) is benecial because it reduces the cost of photocatalysis process. Although different ferrites have been reported in water treatment, [23][24][25] there is a little report on neodymium ferrite (NdFe 2 O 4 ). Nd is a reactive lanthanide with excellent properties that may improve the performance of iron oxide when made into ferrite.
NdFe 2 O 4 is chosen in this study because of its photosensitivity and ideal size, which qualies it as a photocatalyst. Moreover, it is stable and can be cheaply produced. Unfortunately, the particles can agglomerate because of their size, which reduces their efficiency. To overcome the challenge of agglomeration, NdFe 2 O 4 may be incorporated into graphitic carbon nitride (g-C 3 N 4 ) to form NdFe 2 O 4 @g-C 3 N 4 . The g-C 3 N 4 serves as a carbon source to stabilize the particles of NdFe 2 O 4 , preventing agglomeration. Furthermore, g-C 3 N 4 is a carbonbased metal-free semiconductor catalyst that may help boost the catalytic performance of NdFe 2 O 4 . It has a graphite-like layered structure with high chemical and thermal stability, outstanding electronic properties and is non-toxic. [26][27][28][29] Polymeric g-C 3 N 4 possess good photocatalytic property for the degradation of organic pollutants under visible light irradiation. [30][31][32][33] Similar concept of using carbon source material have been previously reported using ZnFe 2 O 4 -carbon allotropes for the photodegradation of noroxacin 34 and ZnFe 2 O 4 @RGO for the degradation of CIP. 35 This study proposes a co-catalytic performance of NdFe 2 O 4 and g-C 3 N 4 in a single entity as NdFe 2 O 4 @g-C 3 N 4 . With the continuous and frequent detection of CIP and AMP in environmental drinking water sources and their incomplete removal during water treatment, this study aims to develop an efficient means for their complete removal in an aqueous system. The study proposes using NdFe 2 O 4 @g-C 3 N 4 as a photocatalyst for removing CIP and AMP in water. From our understanding, there is scanty information on the synthesis and use of NdFe 2 O 4 @g-C 3 N 4 as a catalyst for the photodegradation of CIP and AMP in a water system.
The solution pH (10)(11)(12) was maintained by adding NaOH (2 M) dropwise until precipitate appeared while stirring continued for 2 h. The solution was cooled to room temperature, ltered, and washed severally with C 2 H 5 OH and deionized water until the ltrate was neutral to litmus. The residue was dried at 105°C in the oven for 5 h and transferred to the furnace at 550°C for 18 h.

Preparation of g-C 3 N 4
Briey, melamine (10.00 g) was weight into a crucible and covered appropriately. The covered crucible and its content were transferred into a muffle furnace and heated to 550°C at a controlled heating rate of 5°C min −1 for 3 h. Aer cooling to room temperature, the content of the crucible was poured into HNO 3 (0.5 M) in a 250 mL conical ask and stirred for 1 h. The solution was centrifuged (4200 rcf, 10 min) three times while washing with deionized water. The g-C 3 N 4 obtained was dried at 105°C for 5 h in the oven.

Preparation of NdFe 2 O 4 @g-C 3 N 4 particles
NdFe 2 O 4 (2.00 g) and g-C 3 N 4 (2.00 g) were separately dispersed in deionized water (20.00 mL) in a 100 mL beaker and sonicated for 1 h. Both mixtures were combined and further sonicated for another 1 h. The mixture was transferred to an autoclave and hydrothermally treated (200°C, 6 h). It was cooled to room temperature and centrifuged (4200 rcf, 10 min) twice while washing with deionized water. The solid mass obtained was dried at 105°C for 3 h and annealed in a muffle furnace at 400°C (at 1°C min −1 ) for 2 h.

Photocatalytic degradation of CIP and AMP by NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4
The photodegradation evaluation of CIP and AMP by NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 was achieved in the presence of visible light using a solar simulator (Xe, 150 W) possessing lter holder. 25 Test solutions (50 mL) of CIP (5.00 mg L −1 ) and AMP (5.00 mg L −1 ) were separately exposed to simulated visible light irradiation in the presence of NdFe 2 O 4 (0.1 g) or NdFe 2 O 4 @g-C 3 N 4 (0.1 g) in a beaker (100 mL) while stirring continuously at 120 rpm for 180 min. The UV lamp and the test solution were kept 20 cm from each other. Samples were withdrawn at an interval from the test solution to monitor the degradation of CIP or AMP using a UV-visible spectrophotometer (PerkinElmer, Lambda). Photodegradation was conrmed by UV-visible measurements taken at the appropriate wavelength as follows; CIP (l max = 271 nm) and AMP (l max = 420 nm). The impact of process parameters, such as the effect of weight on the photodegradation of CIP and AMP, was determined by varying the weight (0.1 to 0.5 g) of NdFe 2 O 4 @g-C 3 N 4 while the effect of pH was determined by varying the test solutions pH from 2-10. The effect of test solution concentration on the photodegradation process was estimated by varying the concentration of CIP and AMP from 1.00 to 5.00 mg L −1 . A dark experiment was conducted to establish the adsorption-desorption equilibrium to check whether adsorption was taking place along with the photodegradation process. All experimental conditions (concentration, pH, weight, and time) were kept constant during the dark experiment except exposure to visible light (no irradiation). All the experiments were repeated three times, and values were presented as a mean. The degradation efficiency was calculated as follows: where C 0 is the initial concentration of the test solution and C t is the concentration of the test solution at time t. For the dark experiment, the adsorption capacity (q e ) and the percentage removal (% removal) expressed towards CIP and AMP were calculated as follows: where C 0 (mg L −1 ) represents the initial concentration of the test solution, C e (mg L −1 ) is the test solution concentration at equilibrium; the solution volume is presented as V (in litre), the weight (g) of NdFe 2 O 4 or NdFe 2 O 4 @g-C 3 N 4 is given as m, and the adsorption capacity is given as q e (mg g −1 ).

Scavenging of reactive oxygen species
The mechanism of action for the photodegradation of CIP and AMP using NdFe 2 O 4 @g-C 3 N 4 was investigated by evaluating the role of reactive oxygen species (ROS) in the photodegradation process. This was achieved by examining the role of isopropyl alcohol (IPA) as a hydroxyl radical (OHc) scavenger, ammonium oxalate (AO) as a scavenger for a hole (h + ), and chloroform (CH) as a scavenger for superoxide ion radical (cO 2 − ). Each scavenger (1 mM) was included in the test solution while carrying out the photodegradation process. The process parameters such as weight (0.1 g), pH (7.5), time (180 min), and test solution concentration (5.00 mg L −1 ) for the photodegradation (with and without scavengers) were kept constant.

Regeneration for reuse and stability of NdFe 2 O 4 @g-C 3 N 4
At the end of the photodegradation process, NdFe 2 O 4 @g-C 3 N 4 was recovered from the solution by ltration and washed with solvent (deionized water, 0.1 M HCl, C 2 H 5 OH or mixture of C 2 H 5 OH and 0.1 M HCl (3 : 2)) to regenerate it for reuse. It was oven dried at 105°C for 5 h before being reused for the photodegradation of CIP or AMP. The treated test solution was subjected to inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis to check whether there was elemental leaching of NdFe 2 O 4 @g-C 3 N 4 into the treated test solution during the photodegradation process. Analysis with ICP-OES was conducted at the end of each treatment cycle with NdFe 2 O 4 @g-C 3 N 4 . Furthermore, FTIR analysis was conducted at the end of the treatment cycle on the test solution to check for the presence of organic molecules. The photostability of NdFe 2 O 4 @g-C 3 N 4 for the photodegradation of CIP and AMP was evaluated in een (15) successive cycles of operation. At the end of the 15th cycle, the NdFe 2 O 4 @g-C 3 N 4 was analyzed for XRD and FTIR to check for changes in the structural identity of NdFe 2 O 4 @g-C 3 N 4 .

Results and discussion
from eqn (4) Fig. 1c. The spectra revealed three major mass losses in NdFe 2 O 4 and ve signicant mass loses in NdFe 2 O 4 @g-C 3 N 4 . The mass loss at 60 to 155°C in NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 (rst loss) may be attributed to the loss of adsorbed molecules like H 2 O, CO 2 , and other volatile molecules. The observed loss from 155 to 700°C in NdFe 2 O 4 , which is the second loss, may be attributed to mass loss due to dehydration of the OH group in its structure, involving intra and intermolecular reactions and formation of metal oxides. 47,48 Mass loss from above 700°C in NdFe 2 O 4 (third loss) may be attributed to phase change. Furthermore, the mass loss from 150 to 200°C (second loss) and 200 to 380°C (third loss) in NdFe 2 O 4 @g-C 3 N 4 may be assigned to the decomposition of Fe(OH) 3 to FeOOH and FeOOH to g-Fe 2 O 3 , respectively as previously reported. 38 The mass loss from 380 to 600°C (fourth loss) in NdFe 2 O 4 @g-C 3 N 4 was attributed to the decomposition of the g-C 3 N 4 structure. 49 There is a gradual stable change in mass from 600 to 800°C in NdFe 2 O 4 @g-C 3 N 4 which may be due to the gradual decomposition of g-C 3 N 4 structure to N 2 , (CN) 2 , NH 3 and HCN. 50,51 Fig. 1d revealed the response of NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 to visible light absorption. The spectra showed the absorption region for NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 in the visible light area, which suggest them as potential material with activity within the visible light region. This activity is the reason for proposing the materials for photocatalytic degradation of CIP and AMP. Since these materials are active in the visible light region, their use as photocatalysts will not incur an additional cost since visible light is readily available at no cost, unlike in the case of materials that are only photoactive in the UV region which requires additional cost of incurring UV light source.
The Tauc plot for NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 are shown in Fig. 2a and b, respectively. Their optical band gap was described as follows: where hv is the frequency of the irradiated light, A represents the proportionality constant, E g is the bandgap, and a is the absorption coefficient. The bandgap is 2.10 and 1.98 eV for NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 , respectively. There was a reduction in the bandgap when g-C 3 N 4 was included in the structure of NdFe 2 O 4 to produce NdFe 2 O 4 @g-C 3 N 4 . The bandgap reduction indicates better absorption of visible light by NdFe 2 O 4 @g-C 3 N 4 for enhanced photocatalytic performance. The TEM images of NdFe 2 O 4 (Fig. 2c) and NdFe 2 O 4 @g-C 3 N 4 (Fig. 2d) revealed an average particle size of 14.10 nm and 18.23 nm, respectively. The particles are oval-shaped and arranged in a consistent regular pattern. The surface morphology revealed from SEM images showed the surfaces of NdFe 2 O 4 (Fig. 3a) and NdFe 2 O 4 @g-C 3 N 4 (Fig. 3b) to be heterogeneous, with irregular-sized particles suggesting agglomeration at the surfaces. The particles are stacked, but the surface of NdFe 2 O 4 appeared grainier than that of NdFe 2 O 4 @g-C 3 N 4 , which may be due to the larger crystallite size of NdFe 2 O 4 @g-C 3 N 4 . The elemental mapping of NdFe 2 O 4 (Fig. 3c) and NdFe 2 -O 4 @g-C 3 N 4 (Fig. 3d) revealed the presence of Nd, Fe and O in both NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 whereas C and N were additionally seen in NdFe 2 O 4 @g-C 3 N 4 . The EDS results (Fig. 3e) showed peaks corresponding to the elemental composition of NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 , which further corroborates the elemental mapping.

Photodegradation study
A preliminary investigation of the degradation efficiency of NdFe 2 O 4 and NdFe 2 O 4 @g-C 3 N 4 (Fig. 4a) Fig. 4b. The degradation was progressive over time until it reached equilibrium. However, the initial degradation capacity was highest for the lowest concentration (1.00 mg L −1 ), which may be because the amount of species of CIP in the test solution at the study concentration (1.00 mg L −1 ) was lower than the other concentrations studied and were (CIP species in solution) quickly and rapidly degraded within the rst few periods of the degradation process. Furthermore, the 100% degradation of CIP was achieved at the 80 min of degradation process when the concentration was 1.00 mg L −1 whereas complete degradation was attained at 120 min when the concentration was 5.00 mg L −1 . A similar observation was obtained for AMP (Fig. 4c). Furthermore, a 100% degradation of AMP was only attained at the lowest concentration (1.00 mg L −1 ) studied. As the concentration of the test solution increased, degradation efficiency dropped. For example, at the highest concentration (5.00 mg L −1 ) investigated for AMP, the degradation efficiency obtained is 96 ± 0.80%, whereas, at the least concentration (1.00 mg L −1 ) studied, the degradation efficiency is 100%. This observation is clearly described in Fig. 4d, which shows the decrease in degradation efficiency expressed towards AMP as concentration increased from 1.00 to 5.00 mg L −1 . This observation may be attributed to the fact that as concentration increased, more species of the antibiotics were available in the solution requiring more capacity of NdFe 2 O 4 @g-C 3 N 4 for the degradation process. The effect of NdFe 2 O 4 @g-C 3 N 4 weight on the degradation process revealed that increasing the weight of NdFe 2 O 4 @g-C 3 N 4 from 0.01 to 0.2 favoured the degradation process as the capacity of NdFe 2 O 4 @g-C 3 N 4 to degrade CIP and AMP was enhanced with increase in weight, which may be attributed to the increase in active surface area for photodegradation process. Interestingly, similar degradation efficiency was obtained for 0.1 and 0.2 g weights of NdFe 2 O 4 @g-C 3 N 4 . However, increasing the weight of NdFe 2 O 4 @g-C 3 N 4 above 0.2 g resulted in a decrease in the performance of NdFe 2 O 4 @g-C 3 N 4 . This observation may be because as weight increased beyond 0.2 g, a lesser amount of visible light was able to penetrate the solution for effective photoactivation of NdFe 2 O 4 @g-C 3 N 4 , such may result in a shielding effect which obstructs the excitation of NdFe 2 O 4 @g-C 3 N 4 due to excessive scattering of the photon; similar observation has been reported previously. 53 The test solution pH of CIP and AMP were varied from 2 to 12 while maintaining a concentration of 5.00 mg L −1 , weight of 0.1 g and degradation time of 180 min to investigate the effect of pH on the degradation efficiency of NdFe 2 O 4 @g-C 3 N 4 . Result obtained is presented in Fig. 5b. The degradation efficiency of NdFe 2 -O 4 @g-C 3 N 4 increased as pH increased towards pH 7. NdFe 2 -O 4 @g-C 3 N 4 performed better as pH tends towards neutral, suggesting that more ROS were generated in the test solution at this pH for the degradation process. The performance of NdFe 2 O 4 @g-C 3 N 4 decreased as pH tends towards alkaline pH, suggesting a decrease in the amount of ROS generated. A dark experiment was conducted to understand whether adsorption signicantly affected the photodegradation of CIP and AMP by NdFe 2 O 4 @g-C 3 N 4 . The percentage removal expressed towards CIP is 4.90 ± 0.20% (Fig. 5c), while that of AMP is 4.90 ± 0.50% (Fig. 5d). The percentage removal for the dark experiment increased with time with an adsorption capacity of 2.45 mg g −1 expressed by NdFe 2 O 4 @g-C 3 N 4 towards CIP and AMP. Both photocatalysis and adsorption took place at the same time. However, the contribution from adsorption is about 5% of the total performance demonstrated by NdFe 2 O 4 @g-C 3 N 4 for removing CIP and AMP from the solution, suggesting that about 95% of the contribution made by NdFe 2 O 4 @g-C 3 N 4 towards the removal of CIP and AMP from solution was via photocatalysis.
Data generated from the photodegradation were tted for the pseudo-rst-order kinetic model to better understand the photodegradation process of CIP and AMP using NdFe 2 O 4 @g-C 3 N 4 , which may be expressed as: where C 0 and C t are the initial concentrations of CIP and AMP and concentrations of CIP and AMP at a time "t", respectively, K is the pseudo-rst-order rate constant obtainable from the slope of the plot of ln C 0 /C t versus time and t is the visible light irradiation time. The plot of ln C 0 /C t versus irradiation time at the different concentrations of CIP and AMP are shown in Fig. 6a and b. The rate constant obtained for the degradation of CIP at different concentrations increased with a decrease in concentration (5.00 mg L −1 = 0.0284 min −1 , 4.00 mg L −1 = 0.0286 min −1 , 3.00 mg L −1 = 0.0290 min −1 , 2.00 mg L −1 =  0.031 min −1 and 1.00 mg L −1 = 0.0497 min −1 ). This suggests that the lower the concentration, the faster the degradation process exhibited by NdFe 2 O 4 @g-C 3 N 4 . Similar result was obtained for AMP (5.00 mg L −1 = 0.0251 min −1 , 4.00 mg L −1 = 0.0294 min −1 , 3.00 mg L −1 = 0.0306 min −1 , 2.00 mg L −1 = 0.0340 min −1 and 1.00 mg L −1 = 0.0380 min −1 ). This observation further corroborates the observation in Fig. 4b and c that as concentration reduces, the initial degradation efficiency increases and the highest efficiency was attained with a decrease in the concentration of CIP and AMP.

Proposed photodegradation mechanism of CIP and AMP
Photodegradation of organic molecules may be linked to the generation of ROS, 53 which necessitates investigating ROS's role in the photodegradation exhibited by NdFe 2 O 4 @g-C 3 N 4 towards the removal of CIP and AMP from solution. In order to understand the mechanism of the photodegradation process, the photodegradation of CIP and AMP by NdFe 2 O 4 @g-C 3 N 4 was conducted in the presence of AO (as a scavenger for the hole (h + )), IPA (as hydroxyl radical scavenger) and CH (as a scavenger for superoxide ion radical) as previously mentioned. 25,54 Aer 180 min of photodegradation, it became obvious that AO, IPA, and CH played an important role in the photodegradation of CIP and AMP. The role played by the studied scavengers (Fig. 6c) on the degradation process for CIP is in the order CH (32.50 ± 1.0%) > OA (41.70 ± 0.50%) > IPA (65.80 ± 0.20%) while that of AMP is in the order CH (26.10 ± 0.80%) > OA (33.60 ± 0.20%) > IPA (60.10 ± 0.50%). Therefore, the lower the degradation efficiency obtained, the more the impact of the scavenger on the process. It means that the ROS scavenged has more effect on the degradation process. The highest reduction in degradation efficiency of NdFe 2 O 4 @g-C 3 N 4 towards CIP and AMP was obtained when CH was introduced into the process, suggesting that superoxide ion radical played the most crucial role in the degradation process. It shows that when CH was introduced into the test solution, the superoxide ions in the test solution were scavenged, leading to a reduction in the degradation process. The proposed mechanism for the photodegradation of CIP and AMP by NdFe 2 O 4 @g-C 3 N 4 involving cOH, h + , and cO 2 − (Fig. 6d) suggest the in situ generations of h + and e − from the valence band (VB) and conduction band (CB), respectively, when visible light is absorbed by NdFe 2 O 4 @g-C 3 N 4 . The generated h + reacts with water molecules to produce the H + and cOH while the e − further generates the cO 2 − from the O 2 .
Usually, the h + and e − recombine aer a period, and the degradation process ends. However, if the recombination of h + and e − is avoided, the degradation process will continue. To prevent the recombination of h + and e − generated in situ by NdFe 2 O 4 @g-C 3 N 4 , g-C 3 N 4 was incorporated into NdFe 2 O 4 to serve as a carbon source as well as a co-catalyst. Previous studies have reported a similar concept using carbon dots. 53,55,56 This study used g-C 3 N 4 because it is cheaper to produce, and it is also a catalyst. Therefore, during the photodegradation process, g-C 3 N 4 can serve as an acceptor for trapping h + and e − . Once h + and e − are trapped by g-C 3 N 4 , they are xed at a point which prevents them from interacting. This way, they (h + and e − ) keep generating ROS for a long time to continue the photodegradation process without recombining prematurely. 53,57,58 A recent study demonstrated a plausible mechanism by photoluminescence spectroscopy using terephthalic acid as a probing reagent for monitoring cOH generation. The study revealed the oxidation mechanism for phenol to take place via a Z-scheme unlike the conventional charge-transfer mechanism. 59 During the Z-scheme, e − are excited to the CB leaving a h + in the VB.
The excited e − further migrates to ll corresponding h + , which may reduce H 2 O to H 2 O 2 . Furthermore, the h + may independently oxidize the phenol to CO 2 and H 2 O. The photocatalytic degradation of tetracycline and CIP by p-CaFe 2 O 4 @n-ZnFe 2 O 4 heterojunction has been described to be vis electron-hole interaction, 22 which is similar to the mechanism exhibited in the current study. Furthermore, the mechanism of degradation reported for the degradation of CIP using carbon dot embedded ZnO 53 and Mn/Co composite 60 are similar to the mechanism of ROS generation via electro-hole interaction described for the degradation of AMP and CIP by NdFe 2 O 4 @g-C 3 N 4 .

Regeneration for reuse and stability of NdFe 2 O 4 @g-C 3 N 4
The ability to successfully reuse NdFe 2 O 4 @g-C 3 N 4 for a substantial cycle of treatment without losing functionality is vital. This ability contributes to economic viability and marketability. Therefore, the stability of NdFe 2 O 4 @g-C 3 N 4 for the degradation of CIP and AMP was checked by solvent regeneration for reuse in een cycles. The most suitable solvent for regenerating NdFe 2 O 4 @g-C 3 N 4 was determined as shown in Fig. 6e. Mixture of C 2 H 5 OH : 0.1 M HCl (3 : 2) exhibited better desorption of CIP (98.00 ± 2.00%) and AMP (96 ± 1.00%) from the surface of NdFe 2 O 4 @g-C 3 N 4 than the other solvents used. The regeneration of NdFe 2 O 4 @g-C 3 N 4 aer each treatment cycle was achieved using C 2 H 5 OH : 0.1 M HCl. The regeneration capacity of NdFe 2 O 4 @g-C 3 N 4 for reuse is stable (Fig. 6f). NdFe 2 O 4 @g-C 3 N 4 was stable towards CIP until the 6th cycle of treatment when its capacity dropped from 100.00 ± 1.00% to 98.70 ± 1.00% and at the 10th cycle where it dropped to 97.60 ± 1.00% and remained stable all through the treatment cycles. However, stability expressed towards AMP was stable until the 9th cycle, where it dropped from 96 ± 1.00% to 95 ± 0.60% and remained steady until the 15th cycle of treatment. The stability of NdFe 2 O 4 @g-C 3 N 4 for the photodegradation of CIP and AMP is stable even up to the 15th cycle at a regeneration capacity above 95%, suggesting NdFe 2 O 4 @g-C 3 N 4 to be a promising photocatalyst for the degradation of antibiotics in water treatment. The stability of NdFe 2 O 4 @g-C 3 N 4 was further checked aer the 15th cycle by subjecting it to FTIR and XRD analysis. Fig. 7 revealed that NdFe 2 O 4 @g-C 3 N 4 remains structurally stable even aer the 15th cycle of treatment. Aer the ICP-OES analysis, Nd and Fe were not detected in the treated water. On the other hand, the FTIR did not detect the presence of any organic molecules in the treated water samples for CIP and AMP. Although the degradation efficiency attained for AMP is 96.80 ± 0.80%, the FTIR did not indicate presence of organic molecule in the sample which may be due to low concentration of organic molecules present and being out of the detection limit or sensitivity of the FTIR instrument. Results obtained from the current study were compared with previously reported photocatalysts for the degradation of CIP and AMP, as shown in Table  1. The degradation efficiency exhibited by NdFe 2 O 4 @g-C 3 N 4 for the degradation of CIP is higher than values recently reported for BiVO 4 , 61 Cu 2 O/MoS 2 /rGO, 62 TiO 2 /SnO 2 composite 63 and CeO 2 /g-C 3 N 4 . 64 The regeneration capacity expressed by NdFe 2 -O 4 @g-C 3 N 4 also compares favourably with other photocatalysts. Only ZnO 65 demonstrated similar degradation efficiency to NdFe 2 O 4 @g-C 3 N 4 . However, using ZnO requires a UV source which is an additional process cost that may increase the cost of photodegradation. A previous study using MWCNTs-CuNiFe 2 O 4 exhibited better results than NdFe 2 O 4 @g-C 3 N 4 for the degradation of AMP. 66 However, the degradation efficiency shown by NdFe 2 O 4 @g-C 3 N 4 is higher than 95%, with a remarkable regeneration capacity for reuse. Capacity exhibited by NdFe 2 -O 4 @g-C 3 N 4 towards AMP compared better than studies on La/ Cu/Zr trimetallic, 67 FeSi@MN 68 and ZnO/polyaniline nanocomposite. 69 NdFe 2 O 4 @g-C 3 N 4 has demonstrated the potential to remove CIP and AMP from an aqueous solution. The ease of   synthesis and application of NdFe 2 O 4 @g-C 3 N 4 as a photocatalyst gives it an additional advantage as a resource catalyst with potential in water treatment.

Conclusion
Water pollution with antibiotics is a serious global challenge that requires immediate attention. @g-C 3 N 4 has 100.00 ± 0.00 and 96.80 ± 0.80% for the degradation of CIP and AMP, respectively. Both photodegradation and adsorption were found to be taking place at the same time. NdFe 2 O 4 @g-C 3 N 4 exhibited a stable regeneration capacity for the degradation of CIP and AMP even at the 15th cycle of treatment in a process that can be described by pseudo-rst-order kinetic. The use of NdFe 2 O 4 @g-C 3 N 4 in this study revealed its potential as a promising photocatalyst for removing CIP and AMP in an aqueous solution.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.